1. Field of the Invention
The present invention relates to a switching power supply apparatus (e.g., DC-DC converter), in particular, a switching power supply apparatus having a correction device applicable to a wide input-voltage range with respect to a function of protecting the switching power supply apparatus from overload (i.e., overcurrent).
Priority is claimed on Japanese Patent Application No. 2006-29332, filed Feb. 7, 2006, the content of which is incorporated herein by reference.
2. Description of the Related Art
FIG. 7 is a diagram showing an example of known switching power supply apparatuses (e.g., DC-DC converter), and an overload protection example thereof.
In a switching power supply apparatus 2 in FIG. 7, a DC (direct current) input voltage is applied between power supply terminals VDDin and COM by a DC power supply 10, and a primary winding W1 of a transformer T1, a switching device Q1, and an (electric) current measurement (or detection) resistor R are serially connected between the above power supply terminals VDDin and COM. Switching of the switching device Q1 is controlled by a control circuit CNT.
When the switching device Q1 is switched on, electric current flows in order of the DC power supply 10→the primary winding W1 of the transformer T1→the switching device Q1→the current measurement resistor R→the DC power supply 10. That is, the DC input voltage VDD (which may be simply called the “input voltage VDD” below) is applied to the primary winding W1 of the transformer T1, so that an exciting current having a triangle waveform flows through the primary winding W1 of the transformer T1 and magnetic energy is stored in the transformer T1.
The ON time of the switching device Q1 is determined by a feedback signal from a photocoupler PC. This feedback signal is an error signal obtained by measuring the output voltage Vo on a secondary (winding) side by using an output voltage measurement circuit 21, and comparing the measured voltage with a reference voltage. The feedback signal is transmitted from the secondary side via the photocoupler PC to the control circuit CNT on the primary (winding) side, thereby allowing control for maintaining a constant value of the output voltage Vo on the secondary side.
The photocoupler PC consists of a light emitting diode (LED) PC-D and a light receiving transistor PC-TR.
When the switching device Q1 is switched off, the magnetic energy stored in the transformer T1 is discharged from a secondary winding W2 thereof, via a rectifying diode D21 and a smoothing capacitor C21 to a load 22 on the output side.
A signal output from a tertiary winding W3 is smoothed by a diode D2 and a capacitor C2, and sent as a voltage signal to a terminal Va of the control circuit CNT. To this terminal Va, the input voltage VDD is applied via a resistor R10.
In order to perform overload (or overcurrent) protection, the switching power supply apparatus 2 in FIG. 7 has: the current measurement resistor R connected serially to the switching device Q1; a comparator CMP1 for detecting the overcurrent; and a reference voltage Vref1 provided also for detecting the overcurrent.
A current measurement signal provided by the current measurement using the current measuring resistor R is compared with the reference voltage Vref1 by the comparator CMP1, and when the current measurement signal exceeds the reference voltage Vref1, the comparator CMP1 outputs a disconnect signal for disconnecting (i.e., cutting off) the switching device Q1 to the control circuit CNT. When receiving the disconnect signal, the control circuit CNT outputs a low-level driving signal to a control terminal of the switching device Q1 so as to disconnect the switching device Q1.
However, a series of the above processes, executed from when the current measurement signal exceeds the reference voltage Vref1 to when the switching device Q1 is disconnected, has a slight delay. Accordingly, when the switching device Q1 is disconnected, the electric current which actually flows through the switching device Q1 is higher than that determined based on the reference voltage Vref1. When the input voltage VDD is low, the increase rate of the current which flows through the switching device Q1 is small; thus, the difference between the electric current determined by the reference voltage Vref1 and the electric current which is actually cut off (and is affected by the above delay) is small. However, when the input voltage VDD is high, the increase rate of the current which flows through the switching device Q1 is large; thus, the difference between the electric current determined by the reference voltage Vref1 and the electric current which is actually cut off is large.
As shown by the following formula, the output electric power Wo increases in proportion to the square of the electric current. Therefore, when the input voltage VDD increases, the electric current which is actually cut off increases, and thus the output electric current corresponding to the overcurrent also increases.Wo=(½)L·I2·f 
In the above formula, f indicates the switching frequency.
Therefore, if it is assumed in the switching power supply apparatus that the overcurrent measurement value of the switching device Q1 be fixed, the output characteristics, obtained when the output on the secondary side falls in overload, considerably depend on the input voltage, as shown in FIG. 8, in which the line (a) is obtained when the input voltage is low, while the line (b) is obtained when the input voltage is high.
Consequently, when the input voltage is high, the output current is large; thus, a large stress due to such a current is imposed on the secondary winding and each element of the secondary rectifying circuit. With respect to each element on the primary side, (i) when the switching device Q1 is on, the peak current flowing through the switching device Q1 increases, and (ii) when the switching device Q1 is switched of, the surge voltage generated by the transformer increases due to the increased current, thereby imposing a large voltage stress on the switching device Q1. Therefore, an element having a high rating (with respect to the overcurrent, withstand voltage, and the like) should be employed, which causes an increase in the device size or the manufacturing cost.
In order to solve the above problem, a correction method with respect to the input voltage is known, in which when the input voltage VDD is high, a signal output from an input voltage measurement circuit is superimposed on the signal output from the current measurement circuit, so as to disconnect the switching device Q1 at a smaller overcurrent measurement value.
In the conventional example in FIG. 7, the input voltage VDD is measured using resistors R2 and R3, and a Zener diode D1 and a resistor R1 correspond to a correction circuit (using the input voltage VDD) for the overcurrent protection circuit. The correcting operation using the input voltage in the conventional example will be explained with reference to FIG. 7.
In the following explanation, “VR” indicates a voltage drop (i.e., inter-terminal voltage) of the current measurement resistor R and “VD1” indicates a breakdown voltage of the Zener diode D1. An input voltage VDD1 can be defined by a voltage division between the resistors R2 and R3 and the above breakdown voltage VD1, as follows:VDD1=VD1×(R2+R3)/R3
As a relationship “resistor R1>>resistor R” is generally effective, a voltage at the resistor R provided by the current which flows through the correction circuit (using the input voltage) can be disregarded.
When the input voltage VDD is larger than the above VDD1, a voltage drop VR1 is made to occur at the resistors R1 and R in accordance with the electric current determined by the input voltages VDD and VDD1, and the resistor R2.
That is, VR1 is added to the voltage drop VR of the current measuring resistor R. Therefore, with respect to the comparator CMP1 for detecting the overcurrent, when the input voltage VDD is high, the reference voltage Vref1 is reduced by a value corresponding to the voltage VR1. Accordingly, in the case of the high input voltage, the comparator CMP1 outputs the (switching device) disconnect signal when the current which flows through the switching device Q1 is smaller than that flowing through the switching device Q1 in the case of performing no correction. As a result, the output voltage corresponding to the overload does not depend on the input voltage. In FIG. 8, line (c) indicates characteristics independent on the input voltage, obtained when the correction with respect to the input voltage is applied.
However, this method requires an input voltage measurement circuit for measuring the input voltage VDD, and power consumption of this input voltage measurement circuit cannot be disregarded. In order to further solve this problem, another method for effectively using the tertiary winding of the transformer is known, in which the input voltage is measured using a voltage, which appears on the tertiary winding when the switching device is on, and which depends on the ratio of the primary winding to the tertiary winding, so as to perform the correction with respect to the input voltage (see, for example, Japanese Unexamined Patent Application, First Publication No. 2004-343900).
When a conventional correction method as described above is employed in an integrated circuit a problem occurs, caused by measuring the input voltage and superimposing it on the current measurement value (i.e., signal). More specifically, when the resistor R1 for correcting the input current is employed in an integrated circuit, two input terminal are necessary for the current measurement value and the input voltage correction value. If the correction resistor R1 is provided on the outside of the integrated circuit so as to omit the input terminals, the number of required external parts increases. Additionally, if the input voltage is directly measured, a voltage divider for measuring the input voltage is necessary. On the other hand, when the voltage on a winding of the transformer (i.e., the winding voltage) is measured, a diode is necessary for detecting the winding voltage when the switching device is on, which also increases an external part.